União Biological Reserve, Biological reserve in Rio de Janeiro, Brazil.

União Biological Reserve covers 2,548 hectares across Casimiro de Abreu, Macaé and Rio das Ostras in Rio de Janeiro state. About 58% of the land is hilly coastal terrain with small rounded hills, while 40% is a more rugged zone of ridges and escarpments, plus some flooded areas. It is drained by the Macaé, São João and Ostras river basins, and an exotic eucalyptus stand covers 8.6% of the area.

União Biological Reserve was created on 22 April 1998 and is administered by the Chico Mendes Institute for Biodiversity Conservation (ICMBio). It is classed as an IUCN category Ia strict nature reserve, meant to preserve its biota and natural features without direct human interference. It was established to protect a fragment of Atlantic Forest and the fauna depending on it, especially the golden lion tamarin.

The reserve's name comes from the former União Farm on whose land it was created. Its 249 recorded tree species include areas of marshland and coastal Atlantic Forest in good condition, and an exotic eucalyptus stand covering 8.6% of the area is being replaced by native species. The plumbeous kite visits from September to January.

The reserve lies about 150 km from the city of Rio de Janeiro, within the municipalities of Casimiro de Abreu, Macaé and Rio das Ostras. It is a strict nature reserve (IUCN category Ia) managed by ICMBio, so there are no visitor facilities and entry is not open to the public.

The reserve's 2,548 hectares include about 8.6% planted with exotic eucalyptus, which managers plan to replace with native species. Some 249 tree species have been recorded, and the plumbeous kite visits from September to January.